The invention concerns a tire whose form ratio H/S is at most equal to 0.80, and more particularly a tire of the xe2x80x9cheavy-dutyxe2x80x9d or xe2x80x9ccivil engineeringxe2x80x9d type.
A tire of this kind, which is generally designed to support heavy loads, comprises a radial carcass ply and a crown reinforcement consisting of at least two working crown plies formed of non-extensible reinforcement elements, crossed over from one ply to the next and making with the circumferential direction angles which may be equal or unequal, but which generally range between 10xc2x0 and 45xc2x0.
When the axial width S of such tires is increased without increasing their equatorial and rim seat diameters, i.e. if the height H of the tire on its rim is kept the same, it is found that the contact area on horizontal flat ground changes shape considerably for H/S ratios smaller than 0.65, and this despite the use of a working crown reinforcement that resists the stresses applied to it.
The contact area becomes axially wider but shorter in the longitudinal direction of the tire. This shortening in the longitudinal direction involves an increasingly pronounced retraction in the equatorial zone, to the point when the front and back edges of the contact area may come together, and the said area is then divided into two areas symmetrical with respect to the longitudinal axis of the tread.
Notwithstanding the advantages of wide treads and H/S form ratios smaller than unity, especially in touring vehicles, the unusual shape of the contact area of a tire with form ratio smaller than 0.80 has many disadvantages in the case of tires for heavy vehicles, owing to irregularity in the axial direction of the radial deformations of the carcass reinforcement/working crown reinforcement combination under the action of the stresses due to the inflation pressure.
The said disadvantages are of many kinds, and concern both the durability of the reinforcement armature and the resistance of the tread and the tire to soil aggression (wear, degradation, puncture, impacts, etc.).
To remedy the shortening of the contact area in the longitudinal direction and the resulting lack of durability of the crown reinforcement, patent FR 2 419 185 recommends positioning between the carcass reinforcement and the radially innermost working crown ply, and in two distinct areas of the equatorial plane, two limiting blocks each of which consists of two superimposed plies of non-extensible cables, crossed over from one ply to the next and making opposite angles with the circumferential direction, the absolute value of the said angles being half of the smallest angle used in the working plies and differing by 0xc2x0. Though the solution recommended in the said patent also improves the endurance of the tire since the resistance to separation between the working plies at their edges is increased, while the plies of the limiting blocks are not subjected to the said separation, in contrast it results in excessive fretting which brings the risk that the cables of the working plies may undergo compression when the tire is flattened.
To improve the wear of tires with low H/S values as well as their resistance to crown ply separation, U.S. Pat. No. 4,934,429 claims in contrast to the above that metallic cables can be used, which are extensible and whose orientation relative to the circumferential direction may be zero, the said cables forming part of an additional armature consisting of at least one ply in two portions, one on either side of the equatorial plane, the extensibility of the said cables being chosen as a function of the extension modulus desired respectively for the reinforcement ply or plies and for the crown plies.
The presence of additional armatures of circumferential reinforcement elements or ones very slightly inclined relative to the circumferential direction does not seem to be the best solution to give axial uniformity of the rigidities of the crown reinforcement and at the same time the contact pressures with the ground and/or the slippage between the tread and the ground.
The present invention proposes a different solution for achieving the above uniformity, a solution relating to the constitution and structure of the working crown reinforcement.
The tire according to this invention has a form ratio H/S smaller than 0.80 and comprises a radial carcass reinforcement consisting of at least one ply of non-extensible metallic reinforcement elements. This is covered radially by a working crown reinforcement consisting of at least two working crown plies which are axially continuous and are formed of non-extensible metallic reinforcement elements parallel to one another within each ply and crossed over from one ply to the next, so as to make with the circumferential direction angles xcex1 and xcex1xe2x80x2 between 10xc2x0 and 45xc2x0. The tire is characterized in that the two working plies have axial widths respectively within the ranges 0.65SO to 0.80SO and 0.35SO to 0.45SO where SO is the maximum axial width of the median line of the carcass reinforcement. The working crown reinforcement is completed by the presence of two half-plies consisting of non-extensible metallic elements which, relative to the circumferential direction, make an angle xcex2 smaller than the lesser of the two angles xcex1 and xcex1xe2x80x2 by at least 2xc2x0, the direction of xcex2 being opposite to the direction of the angle xcex1 of the widest axially continuous working ply. The said half-plies are positioned on either side of the equatorial plane and have axial widths between 0.22SO and 0.35SO, such that their axially inteal edges are separated from the equatorial plane by a distance equal to half the width of the narrower working ply, less an amount at least equal to 0.05SO.
It is preferable for the two axially continuous plies formed of elements oriented at angles xcex1 and xcex1xe2x80x2 to be radially closest to the carcass reinforcement and, in this context, the less wide ply is preferably the one closest to the radially outer-most ply of the carcass reinforcement, the said less wide ply having reinforcement elements oriented at an angle said to be xe2x88x92xcex1xe2x80x2 or to the left.
It is advantageous to add, radially outside the working reinforcement defined above and comprising two axially continuous plies of reinforcement elements and two first half-plies, a further two half-plies of non-extensible metallic elements parallel to one another within each half-ply and crossed over with the elements of the two radially lower half-plies making an angle xcex3 with the circumferential direction, the angle xcex3 being on the one hand larger than the largest of the angles xcex1 and xcex1xe2x80x2 of the axially continuous plies and on the other hand also larger than the angle xcex2 of the reinforcing elements of the first two half-plies by at least 10xc2x0.
The axially inside ends of the said two half-plies of elements oriented with the said angle y are separated from the equatorial plane by a distance equal to half the width of the widest axially continuous working ply, less an amount at least equal to 0.05SO.
The axially outer edges of the said half-plies of elements oriented at angle xcex3 may be positioned axially outside the axially outer edges of the two half-plies oriented at angle xcex2, but it is preferable for them to be separated from the equatorial plane by a distance at most equal to the distance between the axially outer edges of the plies oriented at angle xcex2 from the said plane.
The characteristics of the invention will be better understood from the description below, which refers to the drawing illustrating examples of embodiments in a non-limiting way.